Since the invention of the postage meter by Arthur H. Pitney, it has evolved from a completely mechanical postage meter to a meter that incorporates extensive use of electronic components. Postage metering systems have been developed which employ encrypted information that is printed on a mailpiece as part of an indicium evidencing postage payment. The encrypted information includes a postage value for the mailpiece combined with other postal data that relate to the mailpiece and the postage meter printing the indicium. The encrypted information, typically referred to as a digital token or a digital signature, authenticates and protects the integrity of information, including the postage value, imprinted on the mailpiece for later verification of postage payment. Since the digital token incorporates encrypted information relating to the evidencing of postage payment, altering the printed information in an indicium is detectable by standard verification procedures.
Presently, postage metering systems are recognized as either closed or open system devices. In a closed system device, the system functionality is solely dedicated to metering activity. Examples of closed system metering devices include conventional digital and analog postage meters wherein a dedicated printer is securely coupled to a metering or accounting function. In a closed system device, since the printer is securely coupled and dedicated to the meter, printing cannot take place without accounting. In an open system device, the printer is not dedicated to the metering activity. This frees the system functionality for multiple and diverse uses in addition to the metering activity. Examples of open system metering devices include personal computer (PC) based devices with single/multi-tasking operating systems, multi-user applications and digital printers. An open system metering device includes a non-dedicated printer that is not securely coupled to a secure accounting module. An open system indicium printed by the non-dedicated printer is made secure by including addressee information in the encrypted evidence of postage printed on the mailpiece for subsequent verification.
The United States Postal Service (“USPS”) has approved personal computer (PC) postage metering systems as part of the USPS Information-Based Indicia Program (“IBIP”). The IBIP is a distributed trusted system which is a PC based metering system that is meant to augment existing postage meters using new evidence of postage payment known as information-based indicia. The program relies on digital signature techniques to produce for each mailpiece an indicium whose origin can be authenticated and content cannot be modified. The IBIP requires printing a large, high density, two-dimensional (“2-D”) bar code on a mailpiece. The 2-D bar code, which encodes information, is signed with a digital signature. A published draft specification, entitled “IBIP PERFORMANCE CRITERIA FOR INFORMATION-BASED INDICIA AND SECURITY ARCHITECTURE FOR OPEN IBI POSTAGE METERING SYSTEMS (PCIBI-O),” dated Apr. 26, 1999, defines the proposed requirements for a new indicium that will be applied to mail being created using IBIP. This specification also defines the proposed requirements for a Postal Security Device (“PSD”) and a host system element (personal computer) of the IBIP. A PSD is a secure processor-based accounting device that is coupled to a personal computer to dispense and account for postage value stored therein to support the creation of a new “information-based” postage postmark or indicium that will be applied to mail being processed using IBIP.
One version of an open metering system, referred to herein as a “virtual meter”, includes a personal computer, referred to as the host PC, without a PSD coupled thereto. The host PC runs client metering applications, but all PSD functions are performed at a Data Center with which the host PC communicates via a network, such as, for example, a Local Area Network (LAN) or the Internet. The PSD functions at the Data Center may be performed in a secure device attached to a computer at the Data Center, or may be performed in the computer itself. The host PC must connect with the Data Center to process transactions such as postage dispensing, meter registration, or meter refills. Transactions are requested by the host PC and sent to the Data Center for remote processing. The transactions are processed centrally at the Data Center and the results are returned to the host PC. Accounting for funds and transaction processing are centralized at the Data Center. Thus, transactions are computed on an “as-needed” basis, and pre-computing any transactions is not performed. The virtual meter, however, does not conform to all the current requirements of the IBIP Specifications. In particular, the IBIP Specifications do not permit PSD functions to be performed at the Data Center.
In conventional closed system mechanical and electronic postage meters, a secure link is required between printing and accounting functions. For postage meters configured with printing and accounting functions performed in a single, secure box, the integrity of the secure box is monitored by periodic inspections of the meters. More recently, digital printing postage meters typically include a digital printer coupled to a PSD, and have removed the need for physical inspection by cryptographically securing the link between the accounting and printing mechanisms. In essence, new digital printing postage meters create a secure point-to-point communication link between the PSD and print head.
There are problems, however, with digital signature based postage metering systems. Such systems proposed by various Posts, such as the IBIP, place a premium on the protection of the cryptographic keys used to create the digital signatures. Any compromise of these keys would allow an attacker to produce indicia that is verifiable but for which no payment has actually been made. Thus, a sophisticated attacker could perpetrate a significant amount of fraud before being detected. Accordingly, these digital signature based postage metering systems require the meters to be physically secure against sophisticated attacks, such as, for example, physical penetration and differential power analysis, that could reveal the cryptographic keys. Complying with such requirements greatly increases the cost of the meters. Additionally, significant processing power is required to perform the cryptographic calculations within the meter, thereby further increasing the cost of the meter.
Another problem with the digital signature based postage metering systems is that the meter contains the cryptographic keys that are used to authenticate all transactions. A meter owner has no stake in protecting this information, and, in fact, a dishonest meter owner has every incentive to attempt to determine the keys stored in his meter, thereby allowing him to produce indicia without actually paying for them. Thus, the digital signature based postage metering systems place the most sensitive information in the least secure environment.
Although virtual meters overcome the problem of placing the cryptographic keys at the customer site by holding them in a data center, there are problems with this arrangement. Specifically, the customer must now be “on-line” to get postage, i.e., the customer must contact the data center to print postage. Additionally, postal requirements, such as the IBIP, require that the addressee information be sent to the data center to generate the indicium. This is inconvenient for the customer, and also has privacy implications relating to mailing lists.